The present invention relates to a nanotube-shaped titania and a process for producing such a nanotube-shaped titania.
Titania has excellent properties such as ultraviolet absorbability, adsorbability and the like, and thus has been widely used as a material for pigments, paints, cosmetics, ultraviolet shielding agents, catalysts, catalyst supports, and various electronics components. Further, recently considerable attention has been paid to the photocatalytic activity and amphipathic properties of titania exhibited when titania absorbs ultraviolet light. It has been confirmed that titania is effective in decomposing hazardous organic substances, eliminating air pollutants, sterilizing, and self-cleaning.
As one of the conventional technologies for improving these properties of titania, the increase of the specific surface area thereof has been studied. Many attempts have been made to increase the specific surface area of titania, for example, by forming titania into nano-scale fine particles, flakes, wires, tubes, or porous bodies.
Titania particularly in the shape of wire, tube, or porous body such as honeycomb structure can decrease the scattering of titania fine particles at grain boundaries occurring when electrons, holes, photons, or composites thereof produced in the titania upon responding to external stimulus such as heat, light, or temperature change propagate through the titania, more than titania in the shape of fine particle or agglomerate thereof. Therefore, a great number of studies have been carried out on the process for producing titania in the shape of wire, tube, or porous body. For example, Kasuga et al. (Patent Document 1 below) reported that nanotube titania is produced by treating titania particles in a highly concentrated alkali. Furthermore, attempts have been made wherein the shape of titania is controlled by allowing titania to coexist with a substance such as a surfactant which functions as a mold, during the growing process of the titania particles (see Non-Patent Documents 1 to 4 below). However, the conventional processes can not attain titania of a precisely formed structure and have a problem in improving the productivity as well.
Alternatively, as one of the processes for producing titania, there is known a process wherein titanium is anodized. Anodization is a process for producing titania wherein titanium is electrochemically oxidized by applying voltage to an anode of titanium and a cathode of any electrically conductive material in an aqueous solution, thereby forming an oxide of titanium, i.e., titania on the surface of the titanium anode. The process has advantages that it can form the titania film more rapidly than conventional processes and uniformly even though the titanium anode has a large area. Recently, Grimes et al. reported that titania nanotube arrays were produced by anodizing a titanium metal in hydrofluoric acid (see Non-Patent Document 5 below). However, this process is satisfactory in productivity and structural uniformity but not satisfactory in aspect ratio defined as the ratio of the length to diameter of the resulting titania, which is less than 6.                (1) Patent Document 1: Japanese Patent Laid-Open Publication No. 10-152323        (2) Non-Patent Document 1: Japanese Journal of Applied Physics, 2001, vol. 31, p. L1775-L1777, by H. Masuda et al        (3) Non-Patent Document 2: Microporous and Mesoporous Materials (U.S.A), 1999, vol. 30, p. 315-319 by D. M. Antonelli,        (4) Non-Patent Document 3: Chemistry of Materials, (U.S.A), 1997, vol. 9, p. 857-862 by R. Martin        (5) Non-Patent Document 4: Chemistry Letters 2000, p. 942-943, by M. Adachi et al        (6) Non-Patent Document 5: Journal of Materials Research, (U.S.A), 2001, vol. 16, p. 3331-3334 by C. A. Grimes        